1. Field of the Invention
The present invention relates generally to the field of electric power distribution and, more particularly, to a method and apparatus for providing smart or intelligent remote terminal units in a distribution automation system.
2. Statement of the Problem
In the electrical power industry, power is first generated and then distributed to a number of customers. In the generation portion of the electrical utilities business, significant electronic control over the generation has been required. For example, power can be conventionally generated from a hydroelectric plant and then delivered over transmission lines to a high voltage substation. Expensive and physically large electronic controls have been installed at such plants and substations to monitor the generation of power, the occurrence of faults, etc. In the electrical power industry, remote terminal units (RTUs) have been utilized to interconnect with the transmission lines to provide information back to a central location. Such RTUs are essential for the integrity of the generation of electrical power. An interruption in the generation of power substantially affects the entire power electrical network and disrupts downstream distribution of power to the customers. RTUs properly installed in the generation portion of a network are critical and mandatory. The cost of installing an average RTU is approximately $75,000.00. See "Shared Use of Remote Terminal Units for Energy Management Systems and Supervisory Control and Data Acquisition Systems", Maeder, Pacific Coast Electrical Association Conference, Mar. 17, 1989. Such complex RTUs are associated with other disadvantages: the systems are typically not standardized, poorly documented and difficult to maintain and operate. Such RTUs are also not well suited for harsh environments.
The distribution portion of a power network, however, has not utilized any degree of sophisticated electronic control and monitoring. The distribution portion of a utilities network would include distribution feeders, distribution substations, underground transformers, pad mounted transformers, etc., with feeder monitors, pole top capacitor banks, and the like for distribution of electrical power to commercial customers, multiple dwelling customers, industrial customers, and for the various governmental and residential customers. The electric power utility industry has traditionally used manual methods for gathering information concerning the quality of the electricity being distributed and concerning the actual distribution of energy. For example, chart recorders have been used at remote locations to record feeder amps and power flow. Also, tone equipment has been connected to simple mechanical multiplexers and telephone lease lines so as to transmit basic information to a centrally located operations center.
An advance with respect to information gathering pertaining to the distribution of electrical energy has been realized with a Distribution Automation System (DAS). In Distribution Automation (DA) a plurality of RTUs are located in the distribution network such as, for example, on pole tops and in distribution substations which are designed to perform a number of tasks. The topic of Distribution Automation is fully discussed in the IEEE tutorial course entitled "Distribution Automation" (1988) by the IEEE Working Group on Distribution Automation. With the availability of microprocessors, lower cost and "smart" RTUs designed for Distribution Automation have become available. See, for example, "Twenty L First Century and Now Comes the Smart Substation", Electrical Systems Design, June, 1989 (pp 42-43).
While the use of microprocessors in data acquisition both for the transmission and distribution portions of the electric power network has been discussed, the integration of electro-mechanical and static devices under control of an overall microprocessor has been difficult and true integration in data acquisition has not yet been recognized as having occurred. See "Microprocessor Applications to Substation Control and Protection" by Kezunovic and Russell, IEEE Computer Applications in Power, October, 1988 (pp 16-20). Hence, a need exists in the DA environment to integrate into one smart DA/RTU as many functions and features as possible.
In FIG. 1 a prior art Supervisor Control And Data Acquisition (SCADA) RTU is shown. As discussed above, the RTU in FIG. 1 has been utilized in the transmission portion of the electric power network and has typically occupied one or two floor cabinets and has typically been installed in high voltage substations or generator plants. The RTU in FIG. 1 includes the RTU control 10 powered by power supply 20 and interconnected to a distribution feeder line 30 through transducers 40. The transducers 40, as shown in FIG. 1, may include a 3 phase voltage transducer, a 3 phase current transducer, a KW-KVAR transducer, a neutral current transducer and a power factor transducer. The transducers 40 in turn are connected to a potential transformer PT 50 and to a current transformer CT 60. One PT 50 and one CT 60 are needed for each of the phases of the distribution feeder 30. 50 is a conventional sensor for sensing voltage and CT 60 is a conventional sensor for sensing current. Line post sensors could also be used to sense current and voltage in place of PTs 50 and CTs 60. The RTU control is interconnected with a modem 70 and a radio 80 in the FIG. 1 environment although any conventional means of communication with the RTU 10 can be utilized. Finally, the RTU control 10 responds to status inputs over lines 12. In FIG. 1, a conventional Overcurrent Fault Detector 90 and Backfeed Detector 92 having a current sensor CVS 94 interconnected with the distribution feeder 30 is shown. Detector 90 senses an overcurrent condition and detector 92 senses backfeed on a reversal in the direction of current on feeder 30. If the current in feeder 30 is in the proper direction, the output of invertor 98 is high and the AND gate 96 will extend any overcurrent detection as a status input 12 to the RTU 10. If the current in feeder 30 is in the reverse direction, the low output of invertor 98 prevents an overcurrent detection from being delivered to the RTU. This feature will be more fully discussed with respect to FIG. 10, supra. The RTU 10 issues control outputs over lines 14 for controlling items such as switches 100 and the like.
A need exists for a DA/RTU that finds application not only in substations, but on pole tops and other locations in the distribution portion of the electric power network. A need exists for a DA/RTU that is small in size, that is inexpensive to manufacture, install and maintain, that is reliable in operation and performance despite severe environmental conditions, that will provide a stable output, i.e., one that will not drift over time, and that maximizes the amount and quality of data acquisition through microprocessor control. A need exists for a Da/RTU that preprocesses as much data as possible so as to send only condensed information over the communication link 80.